Circuits vIrtual ground is way too virtual ???

[jalbers@bsu.edu]

I have been doing some reading about and experimenting with op amps
(LM741) playing with and following and reproducing the results behind
the classic voltage follower, inverting, non-inverting, summing
feedback loops. After satisfying my curiosity with that I decided to
do some of my own experiments with various components in the feedback
loop to see if I could predict what was going to happen. Just when
you think that you understand something and can go out on your own...

I decided to start by introducing a diode and diode-resistor into the
feedback loop. The circuits are at:
https://ilocker.bsu.edu/users/jalbers/WORLD_SHARED/DiodeFeedback.PDF
These circuits are probably not good for anything other than
experimentation and learning. The diagrams show both the predicted
and actual results.

On page 1, the POT is set to +4V and various voltage measurements were
made around the circuit which were different from what I had
predicted. I am troubled by the fact that the voltages at V+ and V-
are so far apart. They should be micro volts apart. Using an open
loop gain Gopen of 20,000 to 200,000 and Vout = Gopen(V+ - V-) the
output Vout should be way more than 4.43V .

On pages 2A and 2B, the POT is set to +4V and -4V and various voltage
measurements were made around the circuit which were different from
what I had predicted. I am again troubled by the V+ and V-
differential. I thought that an OP amp would do everything in its
power to make V+ and V- the same.

As usual, any “constructive” help would be greatly appreciated.
Thanks

 

[Rich Webb]

On Thu, 14 Aug 2008 10:41:13 -0700 (PDT), "jalbers@bsu.edu"
<jalbers@bsu.edu> wrote:

I thought that an OP amp would do everything in its
power to make V+ and V- the same.

Therefore ... it's not in its power. The typical short circuit current
for an LM741 is 25 mA or so but the spec min is down around 10 mA.
That's with the output grounded, balls to the walls. Observe that the
one you're using seems to poop when driving anything over about 5 mA.
Replace your 1K with a 10K and see what happens.

--
Rich Webb Norfolk, VA

 

[jalbers@bsu.edu]

On Aug 14, 2:35 pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:

On Thu, 14 Aug 2008 10:41:13 -0700 (PDT), "jalb...@bsu.edu"

jalb...@bsu.edu> wrote:
I thought that an OP amp would do everything in its
power to make V+ and V- the same.

Therefore ... it's not in its power. The typical short circuit current
for an LM741 is 25 mA or so but the spec min is down around 10 mA.
That's with the output grounded, balls to the walls. Observe that the
one you're using seems to poop when driving anything over about 5 mA.
Replace your 1K with a 10K and see what happens.

--
Rich Webb     Norfolk, VA

Thanks

Switching to a 10K resistor, the values for the circuit on page 1
change to: V- = 4, Vout = 4.55 (slightly lower than predicted but ok)
Switching to a 10K resisor, the values for the circuit on page 2B
change to: V+ = +4, V- = 3.93, and Vout = 4.52 (the V- V+ differential
is still too wide ????)
Switching to a 10K resisor, the values for the circuit on page 2B
change to: V+ = -4, V- = -4.01, and Vout = -3.47 (pretty much the same
as predicted)